This invention relates to skid-steered automotive devices such as tractors, military tanks, and other such vehicles which move on endless tracks, and more particularly relates to improved methods and apparatus for driving and steering such track-type vehicles and the like. Hereinafter, the term track-type vehicle will be intended to include any type of skid-steered vehicle.
It is well known that all track-type vehicles are relatively quite heavy, and that considerable power is therefore required merely to move such a vehicle in a longitudinal direction. Accordingly, most track-type vehicles require either a gasoline-driven or a diesel engine to generate sufficient motive power to meet the requirements of such devices.
It should be noted, however, that the power requirements for operating a track-type vehicle are not due solely to the relatively great mass of this type of equipment. Such devices also require abnormally large amounts of power for steering purposes, in addition to the power required merely to move such vehicles.
More particularly, it is well known that track-type vehicles are steered by selectively slowing or immobilizing one track, whereby the vehicle tends to drag this track along the ground while turning in the intended direction. Therefore, even greater power is required merely to maintain velocity whenever the vehicle is sought to be deflected from its axial course of travel. Otherwise, the vehicle will tend to slow, or even stop, merely because it is turned in one direction or the other, and this creates an additional load or demand for power which is undesirable if both driving and steering is to be accomplished with power derived from a single or common source.
This problem was initially sought to be solved or at least minimized by providing additional hydraulically actuated engines for separately powering the two track-drive mechanisms. Although this solution did tend to eliminate or greatly reduce the demands for power being made on the main engine (hereinafter the "prime mover") by the steering mechanisms in the vehicle, it greatly added to the overall weight of the vehicle.
Although such an arrangement can obviously achieve some savings in power, it is also expensive to install and maintain such auxiliary equipment. Furthermore, this addition not only makes the vehicle more complicated to operate, the addition of such auxiliary equipment adds greatly to the overall weight of the vehicle, and this may offset the savings in power sought to be achieved.
In another attempt to solve this problem of vehicle weight, hydraulically-actuated steering motors have been substituted for the heavier gasoline-driven engines previously incorporated merely to steer the vehicle. A hydraulic engine is well known to have a lower power capability than a gasoline-driven engine of comparable size, however, and therefore provision of a secondary engine merely to provide steering power for the vehicle has often required the addition of transmissions or other speed reduction units for each steering motor, which has also contributed to the overall weight of the vehicle. In addition;, all of this additional equipment tends to increase the overall cost of the vehicle, and to further make the vehicle more complicated to both operate and maintain.
In order to simplify operation of the vehicle, there have been arrangements designed to synchronize the two steering systems whereby one track-drive mechanism is slowed or stopped whenever the other driving mechanism is accelerated. Although such a modification does operate to achieve its intended purpose, it also tends to limit the degree of control which the operator has over the vehicle, and it further adds to the cost of the vehicle.
In other arrangements, separate braking systems have been provided for both track-drive mechanisms, whereby the operator is given greater discretionary control of the vehicle.
Under many working conditions tractors with conventional clutch-and-brake steering mechanisms cannot be maneuvered to perform useful work, because whenever one of the steering clutches is disengaged the power to that side of the tractor is effectively interrupted. The other side of the tractor then has to pull the entire load in order to maintain momentum, and in many cases the soil strength under one track is inadequate to carry the total load. When this situation is encountered, the powered track tends to shear the soil and to cause the vehicle to lose traction.
A suitable braking mechanism will obviously give the operator greater control over the vehicle. However, braking systems inherently present a maintenance and repair problem, and provision of a separate braking system for each track mechanism will also add to the overall complexity and cost of the vehicle.
In another arrangement, geared steering mechanisms are are provided whereby one track is driven at a lower speed than the other, by means of gear sets in the drive train, rather than disconnecting the steering motor or track mechanism from the drive train for the track sought to be slowed or stopped. However an arrangement of this type is both complex and costly, because it requires duplicate planetary sets, gears, brakes and/or clutches for each drive train.
Various other arrangements have, from time-to-time been proposed to improve the steering capabilites of track-type vehicles. Notwithstanding, whenever a remedy has been proposed which is effective to ameliorate one or more of the problems, the remedy has tended to aggravate an old problem or to create one or more other problems or disadvantages.
These limitations and disadvantages of the prior art are overcome with the present advantage, however, and improved steering methods and apparatus are provided which eliminate the necessity for conventional speed-reduction or transmission units, which further eliminate the necessity for separate clutches or or braking systems for either track-drive mechanism, and which nevertheless provide the operator with full control of the vehicle vehicle concurrent with a reduction in the power requirements and overall weight of the vehicle.